Velocity Limits in the Thermonuclear Supernova Ejection Scenario for Hypervelocity Stars and the Origin of US 708 P
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Astronomy & Astrophysics manuscript no. paper c ESO 2020 June 25, 2020 Velocity limits in the thermonuclear supernova ejection scenario for hypervelocity stars and the origin of US 708 P. Neunteufel1;2 1Max Planck Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München 2University of Leicester, University Road, LE1 7RH Leicester, Leicestershire e-mail: [email protected] Received (month) (day), (year); accepted (month) (day), (year) ABSTRACT Context. Hypervelocity stars (HVS) are a class of stars moving at high enough velocities to be gravitationally unbound from the Galaxy. In recent years, ejection from a close binary system in which one of the components undergoes a thermonuclear supernova (SN) has emerged as a promising candidate production mechanism for the least massive specimens of this class. The explosion mechanisms leading to thermonuclear supernovae, which include the important Type Ia, and related subtypes, remain unclear. Aims. This study presents a thorough theoretical analysis of candidate progenitor systems of thermonuclear SNe in the single de- generate helium donor scenario in the relevant parameter space leading to the ejection of HVS. The primary goal is investigation of the, previously unclear, characteristics of the velocity spectra of the ejected component, including possible maxima and minima, con- straints arising from stellar evolution and initial masses. Further, the question of whether knowledge of the ejection velocity spectra may aid in reconstruction of the terminal state of the supernova progenitor, is addressed, with a focus on the observed object US 708. Methods. Presented are the results of 390 binary model sequences computed with the Modules for Experiments in Stellar Astrophysics (MESA) framework, investigating the evolution of supernova progenitors composed of a helium-rich hot subdwarf and a accreting white dwarf. Assumption of a specific explosion mechanism is avoided as far as possible. The detailed evolution of the donor star as well as gravitational wave radiation and mass transfer driven orbital evolution are fully taken into account. Results are then correlated with an idealized kinematic analysis of the observed object US 708. Results. It is seen that the ejection velocity spectra reach a maximum in the range 0:19 M < MHVS < 0:25 M . Depending on the local Galactic potential, all donors below 0:4 M are expected to become HVS. The single degenerate helium donor channel is able to account for runaway velocities up to ∼ 1150 km s−1 with a Chandrasekhar mass accretor, exceeding 1200 km s−1 if super- Chandrasekhar mass detonations are taken into account. It is found that the previously assumed mass of 0:3 M for US 708, combined with more recently obtained proper motions, favor a sub-Chandrasekhar mass explosion with a terminal WD mass between 1:1 M and 1:2 M , while a Chandrasekhar mass explosion requires a mass of > 0:34 M for US 708. This mechanism may be a source of isolated runaway extremely low mass white dwarfs. Conclusions. The presence of clear, terminal accretor mass dependent, but initial-condition independent, ejection velocity maxima, provides constraints on the terminal state of a supernova progenitor. Depending on the accuracy of astrometry, it is possible to discern certain types of explosion mechanisms from the inferred ejection velocities alone, with current proper motions allowing for a sub- Chandrasekhar mass SN as an origin of US 708. However, more robust reconstructions of the most likely SN progenitor state will require a greater number of observed objects than are currently available. Key words. supernovae: general – white dwarfs – (Stars:) binaries (including multiple): close – (Stars:) subdwarfs – (Stars:) white dwarfs 1. Introduction led to a targeted search, conducted by Brown et al. (2007), which yielded nine further objects in short order, growing to 23 The existence of stars moving at velocities high enough to be objects within the following decade (Brown 2015). unbound from the Galaxy, known as hypervelocity stars (HVS), Theoretical modeling of the ejection mechanism at this point was first proposed more than three decades ago by Hills (1988). in time focused heavily on the black hole connection. Bromley arXiv:2006.11427v2 [astro-ph.SR] 24 Jun 2020 While this initial prediction was followed up by a number of et al. (2006) predicted the likely ejection velocity spectrum of theoretical studies (Hills 1991, 1992), primarily based on the as- HVS produced by the encounter of a binary star with a super sumption that these object would result from interaction of a star massive black hole (SMBH) at the Galactic center (Sag A∗). or binary star with a (Yu & Tremaine 2003), massive black hole O’Leary & Loeb (2008) pointed out that an encounter with a (MBH), observational evidence was not forthcoming. stellar mass black hole in orbit around Sag A∗ could also act as a This changed with the discovery of SDSS source of HVS with Sesana et al. (2009) coming to the same con- J090745.0+024507 by Brown et al. (2005). Found in the clusion regarding intermediate mass black holes (IMBH). The Sloan Digital Sky Survey data set, this object, also referred to latter mechanism has been suggested as a source of certain B- as HVS1, is likely a B-type star with a mass of about 3 M at a type HVS (Irrgang et al. 2019). Very recently, discovery of an distance of 111 kpc from the Galactic center and a Galactic rest A-type star of 2:35 M , moving at a Galactic rest frame velocity frame velocity of 696 km s−1 (Brown et al. 2007). This object of ∼ 1700 km s−1, and which could be traced with high accuracy Article number, page 1 of 17 A&A proofs: manuscript no. paper back to the Galactic center (Koposov et al. 2020), has greatly available for close white dwarf binary systems, including initial bolstered the viability of the SMBH-encounter scenario. For a mass and mass ratio, as well as initial orbital separation, pro- review of the state of the art on this ejection mechanism, the posed to allow for the occurrence of thermonuclear SNe. As the reader is directed to Brown (2015). precise explosion mechanism of these events is currently still Alternative ejection mechanisms to black hole interaction unresolved, the methodology aims to remain agnostic to it and, started being considered in 2009 when Abadi et al. (2009) pro- therefore, to the precise state of the accretor at the time of cre- posed, based on the spatial distribution of known HVS, an ori- ation of the HVS. This paper presents the results of 390 detailed gin in multiple body interaction during the passing of a satellite binary evolution models sequences, commenting on the ejection dwarf galaxy through the Milky Way. The proposal by Justham velocity spectra of a variety of proposed explosion mechanisms et al. (2009) that some HVS could be former members of close of thermonuclear SNe, the pre-explosion evolution of the system binary system with a white dwarf (WD) companion undergo- and the donor star and the viability of using HVS as probes of ing a supernova (SN) explosion provided a conceptual bridge thermonuclear SN explosion mechanisms. to ejection mechanisms earlier proposed for runaway stars (RS) It should be emphasized that the focus of this study is HVS gravitationally bound to the Galaxy (Blaauw 1961; Hoogerw- ejection by thermonuclear SNe in general, of which Type Ia SNe erf et al. 2001), earlier considered to be incapable of providing are considered a subtype, not Type Ia SNe exclusively, whose the necessary ejection velocities. This idea was later followed relatively similar peak luminosity hints at a likewise relatively up theoretically for both B and G/K-dwarf type stars in core col- similar terminal WD mass at the point of explosion (Phillips lapse SNe (Tauris 2015) and hot subdwarfs in the thermonuclear 1993). However, in the absence of a consensus on the spectral scenario (Neunteufel et al. 2016, 2019). classification of hypothetical progenitor SNe of observed HVS, Observational evidence for the viability of the supernova it is reasonable to assume that WD masses at the point of deto- ejection scenario came when Geier et al. (2015) pointed out nation in these events may be dissimilar to those responsible for that the, previously known (Hirsch et al. 2005), helium sdO, of Type Ia SNe. This paper can make no statement on the obser- N-type, indicating unusually fast rotation, US 708 (HVS2) was vational properties of the SN event, only on the velocity of the moving with a greater velocity than originally reported and could expected runaway for a certain assumed WD mass. not be traced back to the Galactic center. This report is organized as follows: Sec. 2 presents a brief re- More recently, accompanying observational successes (e.g. view of the literature concerning explosion mechanisms of ther- Raddi et al. 2019), theoretical interest has focused on the evo- monuclear SNe. In Sec. 3, a number of analytical considera- lution of an SN-ejected HVS in the phase after the supernova tions relevant for the investigation of close binary systems are explosion (Zhang et al. 2019; Bauer et al. 2019), both under the presented. Sec. 4 comments on the numerical tools used in this assumption of the HVS being the former donor star and or a par- project and justifies the choice of initial model parameters. Sec. 5 tially burnt remnant of the former accretor. presents the study’s findings, commenting on the bulk properties With the advent of the latest generation of large scale astro- of the sample and observational properties of certain individual metric surveys (Gaia Collaboration et al. 2018), a number of new cases. Sec. 6 presents a simple application to the observed object objects (Shen et al.